1. Field of the Invention
This invention relates to a solid-state image pickup device, a process for manufacturing the device and a method of driving the device.
2. Description of the Prior Art
In recent years, solid-state image pickup devices making use of charge transfer systems as typified by a charge-coupled device (hereinafter "CCD") have been markedly put into practical use in view of their low-noise characteristics.
A structure commonly used in a conventional solid-state image pickup device will be described below with reference to accompanying drawings.
FIG. 24 is a plan view of what is called a CCD type solid-state image pickup device. The solid-state image pickup device is comprised of a photoelectric transducer area 1, a vertical CCD transfer electrode area 2, a horizontal CCD transfer electrode area 3 and an output area 4.
Upon incidence of light emitted from an object on the photoelectric transducer area 1, electron-hole pairs are produced in the photoelectric transducer area 1 by the action of light. The electrons thus produced are sent from the photoelectric transducer area 1 to the vertical CCD transfer electrode area 2. Into the vertical CCD transfer electrode area 2, electrons of photoelectric transducer areas 1 arranged in the longitudinal direction of the vertical CCD transfer electrode area 2 are sent at the same time.
Then the electrons taken out to the vertical CCD transfer electrode area 2 are sent to the horizontal CCD transfer electrode area 3. Into the horizontal CCD transfer electrode area 3, electrons of vertical CCD transfer electrode areas 2 arranged perpendicularly to the longitudinal direction of the horizontal CCD transfer electrode area 3 are sent at the same time.
The electrons thus taken out to the horizontal CCD transfer electrode area 3 are outputted through the output area 4. Output signals thus obtained pass through an image reproducing circuit and are outputted as an image of the object from an output medium such as a display device.
FIG. 25 show a cross section of the solid-state image pickup device along the line 25--25 (in FIG. 24) that passes across the photoelectric transducer areas 1 and the vertical CCD transfer electrode areas 2.
In the upper layer portion or main surface of an n-type substrate 5, a p-layer 6 having depth and density in given ranges is formed. A photoelectric transducer, an n-layer 7, is formed in a given region in the interior of the p-layer 6. A p-layer 8 with a high density is also formed in the p-layer 6 at a position spaced apart from the n-layer 7. An n-layer 9 that serves as a vertical CCD transfer channel is provided in the p-layer 8.
Signal charges accumulated in the n-layer 7 are read out to the n-layer 9 and then the signal charges are transferred within the n-layer 9 and in the direction perpendicular to the paper face of the drawing.
A high-density p-layer 10 formed on the n-layer 7 is formed so that any dark current caused by an interfacial level of Si-SiO.sub.2 can be prevented from occurring.
Across the p-layer 6 and a lower layer portion 5a (or a region where none of p-layers and n-layers are formed; hereinafter "substrate 5a") of the substrate 5, a reverse bias voltage Vsub that gives positive potential to the substrate and gives negative potential to the p-layer 6 is applied by means of an electric source 11. That is to say, the reverse bias voltage Vsub is applied across the p-layer 8 and the substrate 5a. Hence, the p-layer 6 located below the photoelectric transducer formed of the n-layer 7 becomes depleted. As a result of depletion of the p-layer 6, charges that have turned excessive with respect to the quantity of the charges that can be accumulated in the n-layer 7 are released to the substrate 5a side. It is designed to prevent the blooming phenomenon in this way.
Application of a pulse voltage to the n-type substrate 5 makes it also possible to release all the signal charges in the n-layer 7 to the substrate 5a side, that is to achieve the operation of what is called an electronic shutter.
An insulating film 12 comprising SiO.sub.2 or the like is formed on the surface of the substrate 5. On the insulating film 12, a vertical CCD transfer electrode 13 is formed above the p-layer 8 and n-layer 9 that constitute the vertical CCD transfer electrode area 2 and also above the region embracing a gap region between the n-layer 7 and the p-layer 8. The insulating film 12 is also formed on the side wall and upper surface of the transfer electrode 13 for the purpose of protecting the solid-state image pickup device from a physical impact or shock.
The vertical CCD transfer electrode 13 acts as an electrode for making the electrons accumulated in the n-layer 7 read out to the p-layer 8 or n-layer 9 that forms or serves as the vertical CCD transfer channel.
Thus, in order to prevent the blooming phenomenon or achieve the operation as an electronic shutter, the p-layer 6 must have density and depth in given ranges.
FIG. 26 shows impurity density distribution examined along the line 26--26 passing across the photoelectric transducer area 1 in FIG. 25.
FIG. 27 shows net values of the impurity densities shown in FIG. 26.
In both FIGS. 26 and 27, the impurity densities are plotted as ordinate, and the distance from the substrate surface is plotted as abscissa. The densities of p-type impurity atoms are indicated on the upper side of the ordinate and the densities of n-type impurity atoms are indicated on the lower side of the ordinate. Broken lines 14, 15, 16 and 17 in FIG. 26 represent the impurity densities of the p-layer 10, p-layer 6, substrate 5a and n-layer 7 in FIG. 25, respectively.
A solid line 18 in FIG. 27 represents the net values of the respective impurity densities shown in FIG. 26. The shaded region 19 corresponds to the net impurity density distribution of the photoelectric transducer n-layer 7, showing the amount of effective donors accumulated in the n-layer.
The impurity density 17 of the photoelectric transducer n-layer 7 is highest at the surface of the substrate 5. The density decreases with a depth toward the interior of the substrate 5. The p-layer 10 is formed at the upper part of the n-layer 7 so that the dark current can be decreased. The impurity density 14 of the p-layer 10 is higher than the density of the n-layer 7, and is less spread toward the interior of the substrate 5. Hence, the n-layer 7 comes to have the net impurity density 19.
The p-layer 6 has a low impurity density, but its impurity density is distributed in a broadly spread state. More specifically, its impurity density is at maximum at the surface of the substrate 5 and gradually decreases with a depth toward the interior of the substrate 5.
In the solid-state image pickup device, in order not to cause the blooming phenomenon, it is necessary for the photoelectric transducer n-layer 7 to be almost completely depleted after the electrons accumulated in the photoelectric transducer have been read out to the vertical CCD transfer electrode area. For this reason, the amount of the effective donors represented by the net impurity density 19 must be controlled to be an upper limit of the quantity of the charges that can be accumulated in the n-layer 7. In other words, it follows that the amount of effective donors determines an upper limit value of saturation characteristics of the photoelectric transducer.
The net impurity density 19 is a value obtained by subtracting the impurity density 14 of the p-layer 10 and the impurity density 15 of the p-layer 6 from a value obtained by adding the impurity density 16 of the substrate 5a to the impurity density 17. Because of the restrictions in view of each process, the net impurity density 19 depends on the impurity density 14 and impurity density 17. As to the impurity density 17, the density is highest at the surface of the substrate 5. The part where this density is highest is compensated by the impurity density 14 of the reverse conductivity type p-layer 10.
FIGS. 28 to 31 cross-sectionally illustrate a flow sheet for fabricating a conventional CCD type solid-state image pickup device.
In the main surface (i.e., the upper layer portion) of the n-type substrate 5, p-type impurity boron is ion-implanted. Thereafter, the p-layer 6 is formed by a high-temperature beat treatment. Thereafter, a resist pattern is formed by conventional photolithography on the region other than the region in which the vertical CCD transfer channel is formed. Using the resist pattern as a mask, boron is ion-implanted to form the p-layer 8. Thereafter, a resist pattern is again formed by conventional photolithography on the region other than the region in which the n-layer 9 in the p-layer 8 serving as the vertical CCD transfer channel is formed. Using the resist pattern as a mask, phosphorus is ion-implanted 2 to form the n-layer 9. Thereafter, the insulating film 12 is deposited on the main surface of the substrate 5 by thermal oxidation. An electrode material serving as the vertical CCD transfer electrode 13 is further formed on the insulating film 12 (FIG. 28).
Then, a resist pattern 20 is formed on the electrode material by photolithography on the region other than the region broader than the region that serves for the transfer electrode 13. Using the resist pattern as a mask, the electrode material is dry-etched until the insulating film 12 is uncovered. Next, using the resist pattern 20 as a mask and also making the insulating film 12 serve as a protective film, phosphorus is ion-implanted. The n-layer 7 serving as the photoelectric transducer is formed as a result of this ion implantation (FIGS. 29 to 30).
Thereafter, the resist pattern 20 is removed. On the surface of the substrate 5, an electrode material is formed with a region broader than the insulating film 12 and transfer electrode 13. Next, a resist pattern is formed on the region other than the region in which the transfer electrode 13 is formed. Using this resist pattern as a mask, the electrode material is dry-etched. Through the above process, the transfer electrode 13 is formed. At this time, the transfer electrode 13 must be provided also on the gap region between the n-layer 7, i.e., the photoelectric transducer from which electrons are read out to the vertical CCD transfer channel, and the p-layer 8. For this purpose, since in the dry etching carried out when the transfer electrode 13 is formed a side wall end of the n-layer 7 serving as the photoelectric transducer is originally on the same line with a side wall end of the transfer electrode 13, one side wall end of the transfer electrode 13 is shortened to be formed in the desired length (FIG. 30).
Next, using one end of this transfer electrode 13 and a resist as masks, boron is ion-implanted. The p-layer for preventing dark current is thus formed on the n-layer 7 (FIG. 31).
The conventional solid-state image pickup device as described above has the following disadvantages.
The impurity density 17 is highest at the surface of the substrate 5, and this part where the density is highest is compensated by the impurity density 14 of the reverse conductivity type p-layer 10. Hence, of the impurity density 17 of the n-layer 7 introduced as a photoelectric transducer, the region having a relatively low density is used as the photoelectric transducer. In other words, a photoelectric transducer with a low density results in a small amount of the effective donors that can be accumulated there, and can not achieve the quantity of saturation charges at a sufficiently high level.
Accordingly, the impurities to be lead into the region of impurity density 17 are implanted in an increased quantity. The implanted impurities are also diffused into the depth of the substrate by a high-temperature heat treatment so that the photoelectric transducer can have an increased area.
The high-temperature heat treatment, however, may result in a spread of the impurity density 17 in the direction perpendicular to the surface of the substrate 5. As a result, the impurities are also diffused in the gap region between the p-layer 8 and n-layer 7 and also in the p-layer 8, the vertical CCD transfer channel.
The positional relationship between an end portion of the n-layer 7 and the vertical CCD transfer electrode 13 is determined by the precision for the registration of the mask in the step of exposure carried out when the transfer electrode 13 is formed and also on the diffused region of the n-layer 7 formed by diffusion. For this reason, it is very difficult to control the position at which the n-layer 7 is formed by heat diffusion at a high temperature. A poor controllability for the positions of the end portion of the n-layer 7 and the vertical CCD transfer electrode 13 causes the electrons serving as signals to be left in the photoelectric transducer at the time of read-out where they are taken out from the photoelectric transducer to the vertical CCD transfer channel.
The electrons thus having been left cause the phenomenon of phantom or residual images, resulting in a serious deterioration of picture quality.
On the other hand, when the impurities are implanted in an increased quantity, faults in ion implantation increase to increase what is called white scratches. This results in a lowering of the yield.
Moreover, the p-layer 6 is formed as a single impurity layer. Hence the voltage applied to the p-layer 6 must be controlled in order to prevent the blooming phenomenon or achieve the operation as an electronic shutter.
In the device with such structure, the voltage applied to the n-type substrate 5 is usually controlled to be about 10 V in order to control the blooming phenomenon. A pulse voltage of about 30 V is also required in order to achieve the operation as an electronic shutter.
For the purpose of applying such voltages, the number of parts must be increased, resulting in an increase in power consumption, when camera-combined VTRs usually making use of the solid-state image pickup device are manufactured. This obstructs the manufacture of those being small-sized, lightweight and also of low power consumption.